An oscillator based upon a transformer has two modes of operation. By utilizing both modes, the oscillation range can be doubled. Inductor-capacitor (LC) voltage controlled oscillators (VCOs) have been used for multi-band, multi-standard and broadband applications because of their low phase noise. However, these types of oscillators have a limited oscillation range. Further, two or more inductors are used to achieve twice operation range, but consume high silicon area and are therefore not low cost.
Further, a major issue in transformer-based VCOs is the ability to select the correct mode. Conventional methods have used mode amplitude-based designs to select the appropriate modes. Assuming that an ideal transformer is being implemented, the ratio of two capacitors is selected such that amplitude of one mode becomes higher than that of the other mode. However, unwanted simultaneous oscillation can take place because the phase may still be 0 in both modes, leading to mode ambiguity. Other conventional devices solve the problem associated with mode selection by adding additional (i.e. double driving) cells. However, this solution leads to high power consumption.
Accordingly, circuits and methods of implementing a dual-mode oscillator in an integrated circuit that eliminates unwanted oscillations and reduces power are desired.